In the present day techniques have been developed to grow single crystalline (monocrystalline) sheets from a melt of a given material such as silicon. This is accomplished by crystallizing a thin solid layer of the given material at a given position on a surface of a melt composed of the given material, and pulling the thin solid layer along a pull direction. As the monocrystalline material is drawn along a given direction, a ribbon of monocrystalline material may form in which one end of the ribbon remains stationary at the given position or crystallization region in which crystallization takes place. This crystallization region may define a crystallization front (leading edge) between the monocrystalline sheet and the melt that is defined by a crystal facet that forms at the leading edge.
In order to sustain the growth of this faceted leading edge in a steady-state condition so that the growth speed matches the pull speed of the monocrystalline sheet, or “ribbon,” intense cooling may be applied by a crystallizer in the crystallization region. This may result in formation of a monocrystalline sheet whose initial thickness is commensurate with the intensity of the cooling applied, which initial thickness is often on the order of 1-2 mm in the case of silicon ribbon growth. However, for applications such as solar cells to be formed from a monocrystalline sheet or ribbon, a target thickness may be on the order of 200 μm or less. This requires a reduction in thickness of the initially formed ribbon, which may be accomplished by heating the ribbon over a region of a crucible that contains the melt as the ribbon is pulled in a pulling direction. As the ribbon is drawn through the region while the ribbon is in contact with the melt a given thickness of the ribbon may melt back, thereby reducing the ribbon thickness to a target thickness. This melt-back approach is particularly well suited in the so-called Floating Silicon Method (FSM), which forms a silicon sheet on the surface of a silicon melt according to the procedures generally described above.
However, during growth of a monocrystalline sheet using a method such as FSM, sheet thickness may vary across the width of the monocrystalline sheet, that is, along a transverse direction that is perpendicular to the pull direction. This may vary from run to run, or even within a run, where a run corresponds to a process that produces a single ribbon of monocrystalline material. Additionally, because the final target thickness of a ribbon may be a factor of ten thinner than the initial thickness, precise control of thickness uniformity may be especially important. For example, a device application may specify a substrate thickness of 200 μm+/−20 μm. If a monocrystalline sheet is crystallized that has an initial thickness of 2 mm near the crystallizer and an initial thickness variation of 5% (or 100 μm), without correction of this initial thickness variation, after the ribbon is thinned to 200 μm thickness by drawing the ribbon through a melt-back region, the thickness variation of 100 μm now constitutes a 50% variation in thickness, which may render the ribbon useless for its intended application. Moreover, the thickness of a ribbon may vary along the transverse direction in a manner that is not easily corrected by melting back the ribbon using a conventional heater.
It is with respect to these and other considerations that the present improvements have been needed.